1. Field of the Invention
This invention relates to a tape position marker sensing circuit for sensing EOT and BOT markers on magnetic tape and more particularly to a position marker sensing circuit which controls the energization of an illumination source to maintain the intensity of illumination reflected from a magnetic tape at a reference level.
2. Description of the Prior Art
Digital magnetic tape transports utilize reflective markers positioned a few feet from the beginning and end of a reel of tape to delimit the region along the length of the tape on which information may be written. Sensing of a beginning of tape (BOT) marker indicates that reading or writing may commence in a forward direction and indicates that the initial end of the tape is being approached as tape is being rewound. The sensing of an end of tape (EOT) marker indicates that the end of the tape is approaching and that reading or writing in the forward direction should be terminated.
The reflective EOT and BOT markers are typically about an inch in length and are disposed adjacent the back or nonoxide side of the tape with one being disposed across a first or left hand half of the tape and the second being disposed across a second or right hand half of the tape. A light source is disposed to shine light on a marker sense position along the length of the tape and first and second photosensors are disposed to receive light from the light source which reflects from the left and right half respectively of a tape lying in the tape path. The outputs of the two photosensors are compared to a reference and the photocell signal greater than the reference indicates that a photocell is receiving an increased intensity of reflected light due to the presence of a tape marker at the marker sensing position. An increased intensity at one cell indicates the presence of the marker.
These tape marker sensing circuits must function under a variety of operating conditions which affect circuit operation. One of these conditions is temperature, which tends to affect all circuit components, but has a greatest effect on the intensity of illuminated light where a light emitting diode is utilized as the illumination source. Because of this temperature dependence, a reference level that is selected for one temperature may not be suitable for a substantially different temperature because of uncompensated changes in the light source and photosensors with temperature. As a result, either temperature isolation or temperature compensation must be employed to permit the circuit to operate properly over a wide range of temperatures.
Another operating condition which substantially affects circuit operation is the color or reflective characteristics of magnetic tape which is being sensed by the circuit. A tradeoff must be established in presetting the reference level for best noise immunity for sensing tape position markers. A dark or non-reflective tape provides the best noise immunity and permits the reference level to be set relatively low to permit a reflective marker to be readily sensed. Alternatively, a light or highly reflective tape requires the sensing reference level to be set somewhat higher with an attendant decrease in the noise immunity due to the fact that there is less difference in the sensed intensity of light reflected from the tape and the position markers. If the intensity sensing level is selected to accommodate more reflective tapes, the better noise immunity of less reflective tapes cannot be fully utilized. On the other hand, if the reference is placed at a lower level to utilize the better noise immunity of less reflective tapes, it may not be possible to utilize the circuit with the more highly reflective tapes. Other problems are encountered when minor asymmetry in the actual positioning of the illumination source and sensors results in deviation of the sensed reflected illumination intensity from that which is expected and when long term variations develop in the intensity of emitted illumination.